This invention relates to the inspection of rotor slot bottoms of rotor disks and, more particularly, to a fixture that aids in performing the inspections.
In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is combusted, and the resulting hot combustion gases are passed through a turbine mounted on the same shaft. The flow of gas turns the turbine by contacting an airfoil portion of the turbine blade, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward. There may additionally be a turbofan that drives a bypass flow of air rearwardly to improve the thrust of the engine.
The compressor, the turbine, and the turbofan have a similar construction, in that they each include a rotor disk and a set of removable blades extending radially outwardly from the rotor disk. The rotor disk has a series of rotor slots extending parallel to the axis of revolution of the rotor disk, and the roots of the blades are slidably engaged into the slots. The slots and the roots of the blades are conformably shaped to hold the blades in the rotor disk when the rotor disk rotates about its axis of rotation. In current practice, the rotor slots and the roots of the blades have a conformable dovetail shape. While the compressor, the turbine, and the turbofan share this basic configuration, the materials of construction of the rotor disks and the blades, as well as the shapes and sizes of the rotor disks and the blades, vary in these different sections of the gas turbine engine.
One of the failure modes of the rotor disk is the formation of cracks in the bottoms of the rotor slots. These cracks typically initiate due to a combination of creep and fatigue. One of the cracks eventually may enlarge sufficiently that it propagates and leads to a catastrophic failure of the rotor disk.
To avoid such a failure, the slot bottoms are periodically inspected using visual or fluorescent penetrant inspection (FPI) techniques. These techniques identify cracks at the bottoms of the slots when they are small and before they can grow to a size that they propagate, so that the cracks may be repaired. These techniques, while operable, are subjective in that they depend upon the skill, judgment, and state of mind of the person performing the inspection. They are only semiquantitative in nature, in that the number, length, and type of cracks are subjectively judged. Visual and FPI approaches are also not always reproducible and do not lend themselves to automation of the inspection.
There is accordingly a need for an approach which improves upon the existing inspection techniques. The present invention fulfills this need, and further provides related advantages.
The present invention provides an inspection apparatus and method for inspecting the slot bottoms of rotor slots. The present approach provides for objective analysis of the state of the slot bottoms. It does not depend upon the state of mind of the person performing the test, yields quantitative results, provides for extensive automation of the inspection procedure, and has excellent reproducibility of the inspection.
An inspection apparatus is used in relation to a rotor disk having a plurality of circumferentially adjacent rotor slots therein extending parallel to an axis of revolution of the rotor disk. Each rotor slot has a rotor slot side and a rotor slot bottom with a slot bottom surface. The inspection apparatus comprises an inspection fixture including a base, and at least one guide extending from the base. Each guide is slidably engagable to one of the rotor slots and has a guide side shaped to slidably conform to the rotor slot side. Each guide further has a guide bottom with a guide bottom surface which, in combination with the slot bottom surface, defines an elongated inspection cavity, which is preferably substantially cylindrical with its cylindrical axis extending parallel to the axis of revolution of the rotor disk. Preferably, the inspection fixture comprises more than one guide, each guide as set forth above. The guides are spaced apart and angled so as to slidably engage the respective rotor slots of the rotor disk.
The inspection apparatus may further include a sensor apparatus comprising a sensor sized to slide into the inspection cavity. The sensor is preferably an eddy current sensor. The sensor apparatus typically also includes a sensor drive that moves the sensor parallel to a direction of elongation of the inspection cavity. In the case of the eddy current sensor, the sensor drive rotates the sensor about its sensor axis while in the inspection cavity. Eddy current inspection of the slot bottoms is an important advance over visual and FPI techniques. It may be automated and is not dependent upon operator skill. It produces quantitative results that are reproducible. In the preliminary development by the inventors leading to the present invention, the advantages of eddy current inspection were recognized but could not be achieved because the sensor could not be positioned sufficiently accurately relative to the slot bottoms. The inspection fixture solves this problem, allowing the eddy current sensor to be precisely and reproducibly positioned relative to the slot bottoms during the inspection procedure, so that its full advantages may be achieved.
The inspection apparatus is used to inspect a rotor disk as described above. The method includes providing an inspection apparatus comprising the inspection fixture as described above, and assembling the inspection fixture to the rotor disk such that each guide slides into one of the rotor slots. The sensor apparatus as described above is provided, and the sensor is inserted into the inspection cavity. The rotor slot bottom is sensed using the sensor. Preferably, the sensor drive rotates the eddy current sensor about its sensor axis and also permits moving the eddy current sensor parallel to the direction of elongation of the inspection cavity during the sensing operation.
The inspection fixture precisely positions the sensor of the sensor apparatus. The sensor must be tightly constrained to a close facing contact to the slot bottom surface during the inspection process. If the sensor were allowed to separate from the surface of the slot bottom by even 0.001 inch, the inspection sensitivity would be reduced and defects might be undetected. The inspection fixture, sensor drive, and split-sensor structure cooperate to ensure that the sensor remains in intimate contact with the slot bottom. In the preferred case where the sensor is an eddy current sensor, the intimate contact is maintained as the sensor is rotated about its sensor axis and moved parallel to the sensor axis.
Although a particular embodiment of the invention has been described in detail for purposes of illustration, various modifications and enhancements may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as by the appended claims.